Which pathogen is associated with water sources and classified as a parasite?

Cryptosporidium is a parasite that is well-known for being associated with contaminated water sources. It is a microscopic organism that causes an illness called cryptosporidiosis, which can lead to gastrointestinal symptoms. This parasite is resilient and can survive in harsh environments, making it particularly concerning in water supplies, especially in areas where water treatment may be inadequate.

In contrast, the other options listed are primarily known as bacteria and have different associated transmission routes. Salmonella and E. coli are often linked to contaminated food sources rather than water. Listeria, although it can be found in various environments, is also primarily associated with food products. Thus, the focus on water contamination specifically identifies Cryptosporidium as the correct choice.